Process for producing polyethylene

ABSTRACT

The present invention relates to a process for producing ethylene copolymers in a multistage process comprising at least one slurry phase polymerization stage and at least one gas phase polymerization stage in the presence of Ziegler Natta catalyst comprising a solid catalyst component, a cocatalyst of a compound of group 13 metal and an external additive selected from alkoxysilanes of formula (I) R 1   n Si(OR 2 ) 4-n , (I) where n is an integer 0 to 3, each R 1  are equal or different and are selected among H, halogen, alkyl groups of 1 to 6 10 carbon atoms optionally substituted with one or more halogen atoms, alkenyl groups of 2 to 6 carbon atoms optionally substituted with one or more halogen atoms, and aryl groups of 6 to 12 carbon atoms optionally substituted with one or more halogen atoms, or the R 1  groups can form with the Si atom they are linked to a ring of 3 to 8 ring atoms, provided that all R 1  are not hydrogen, R 2  are equal or different and are selected among alkyl groups of 1 to 6 carbon atoms optionally substituted with one or more halogen atoms, alkenyl groups of 2 to 6 carbon atoms optionally substituted with one or more halogen atoms, and aryl groups of 6 to 12 carbon atoms optionally substituted with one or more halogen atoms, or the OR 2  groups can form with the Si atom they are linked to a ring of 3 to 8 ring atoms, halogen is Br, CI or F. The invention further relates to the catalysts and use thereof in said multistage process r for producing ethylene copolymers having melt flow rate ratio FRR 21/5  at least 40 and/or polydispersity index PDI of at least 27.

This application is a U.S. national stage entry under 35 U.S.C. § 371 ofPCT International Patent Application No. PCT/EP2016/052354, filed Feb.4, 2016, which claims priority to European Patent Application No.15153922.8, filed Feb. 5, 2015, the contents of which are incorporatedherein by reference in their entirety.

This invention relates to a process for producing ethylene polymers in amultistage process and in the presence of Ziegler Natta catalystcomprising a solid catalyst component, Group 13 metal compound ascocatalyst and an external additive. The invention further concerns saidcatalysts and their use in producing ethylene polymers with desiredproperties.

BACKGROUND OF THE INVENTION

Ziegler-Natta (ZN) type catalysts are well known in the field ofproducing polymers from olefinic monomers, like ethylene (co)polymers.Generally the catalysts comprise at least a catalyst component formedfrom a transition metal compound of Group 4 to 6 of the Periodic Table(IUPAC, Nomenclature of Inorganic Chemistry, 1989), a metal compound ofGroup 1 to 3 of the Periodic Table (IUPAC), and, optionally, a compoundof group 13 of the Periodic Table (IUPAC) and/or optionally an internalorganic compound, like an internal electron donor compound. The ZNcatalyst may also comprise further catalyst component(s), such ascocatalyst(s) and optionally external additives, like external donors.

A great variety of Ziegler-Natta catalysts have been developed tofulfill the different demands in reaction characteristics, large-scaleproduction, and producing poly(alpha-olefin) resins of desired physicalperformance. One type of a typical Ziegler-Natta catalyst component ispreferably comprised of a magnesium compound, an aluminium compound anda titanium compound supported on a particulate support. The particulatesupport can be an inorganic oxide support, such as silica, alumina,titania, silica-alumina and silica-titania, typically silica.

The catalyst component can be prepared by sequentially contacting thecarrier with the above mentioned compounds, as described, for example inEP 688794 and WO 99/51646. Alternatively, it can be prepared by firstpreparing a solution from the components and then contacting thesolution with a carrier, as described in WO 01/55230.

Another group of typical Ziegler-Natta catalysts are based on magnesiumdihalide, typically MgCl₂, that contain a titanium compound andoptionally a Group 13 compound, for example, an aluminium compound. Suchcatalysts are disclosed, for instance, in EP376936, WO 2005/118655 andEP 810235.

The above described ZN-catalysts are described to be useful in olefinpolymerisation, i.e. for producing ethylene (co)polymers.

However, even though many catalysts of prior art show satisfactoryproperties for many applications, there has been a need to modify andimprove the properties and performance of the catalysts to achievedesired polymer properties and to have catalysts with desiredperformance in desired polymerisation processes.

Adding various molecules, such as internal organic compounds or externaladditives can influence the polymerization character of the catalyst andthereby the subsequent polymer properties. The internal organiccompounds can be internal electron donors or other compounds havinginfluence on the performance of the catalyst. An example of externaladditives is external electron donors. In the present application, thephrases external electron donor and external additive areinterchangeable, and also internal electron donor and internal organiccompound are interchangeable.

U.S. Pat. No. 5,055,535 discloses a method for controlling the molecularweight distribution (MWD) of polyethylene homopolymers and copolymersusing a ZN catalyst comprising an electron donor selected frommonoethers (e.g. tetrahydrofuran). The monoether, which istetrahydrofuran in this case, is added to the catalytic component withthe cocatalyst, at the latest, upon commencement of the polymerisationreaction, and is further characterised that under no circumstance shouldthe monoether be brought into contact with the catalytic componentwithout the presence of the cocatalyst in the medium.

WO2005058982 discloses a two-stage gas-phase polymerisation process forproducing high density polyethylene (HDPE) in the presence of a solidZiegler-Natta catalyst component and alkylaluminum compound ascocatalyst. Further, an external donor is added into the second gasphase reactor so that the disclosed process is then capable of producinga relatively broad molecular weight ethylene copolymer in the presenceof a Ziegler-Natta catalyst capable of retaining at the same time goodhydrogen sensitivity and a capability to homogeneously distribute thecomonomer. Said external donor can be the same or different to theoptional internal donor, and is preferably an ether, liketetrahydrofuran (THF). Alkoxysilanes are also listed among otherexternal donors, such as alcohols, glycols, ketones, amines, amides andnitriles. The catalyst productivity is not discussed or disclosed, noris any problem relating to the production of high Mw ethylene(co)polymer. In WO2005058982 it is only discussed the possible negativeimpact of external donors on the hydrogen response and consequently onthe activity of the catalyst in the polymerization step, where therelatively low molecular weight the polymer is produced. However,producing high Mw ethylene (co)polymer good hydrogen response of thecatalyst and/or substantial hydrogen carry-over from the reactor inwhich the relatively low molecular weight polymer is produced can causeproblems. Moreover, it is generally known in the art that not each andevery external additive improves comonomer distribution.

WO 2007051607 A1 suggests the possibility of producing a multimodalethylene polymer by using alkyl ether type internal electron donors tomodify the ZN catalyst component. The final molecular weightdistribution (MWD) is narrower due to the reduction of MWD of a highermolecular weight (HMW) component. The electron donor is preferablytetrahydrofuran.

The use of alkoxysilanes as external electron donors with respect topolymerization of α-olefins, particularly, with respect topolymerization of propylene for increasing stereo-regularity/tacticityby Ziegler-Natta catalysts is commonly known in the field and is widelyused in the industry, as described, for example, in U.S. Pat. Nos.4,547,552, 4,562,173, 4,927,797, WO03106512, and EP0303704. In additionto stereo-regularity/tacticity control also other properties of thefinal propylene polymer may be affected by use of an external electrondonor.

Alkoxysilane type external donors are not commonly used nor widelypresented in patent literature in ethylene (co)polymerization. However,WO200238624 discloses that use of specific alkoxysilanes together with ahaloalkane compound in ethylene polymerization in the presence ofcocatalyst and a very specific solid titanium catalyst component resultsin PE with narrow molecular weight distribution and high bulk densitywith high activity. WO200238624 does not discuss polymerisation in amultistage polymerisation process or polymerisation in a gas phasereactor. All polymerisation examples describe one-step liquid-phasepolymerizations.

WO2004055065 discloses a solid catalyst component comprising Ti, Mg,halogen and electron donor in specific molar ratios for the preparationof copolymers of ethylene with α-olefins, where said α-olefins arehomogeneously distributed along the polymer chain. Said catalyst is usedin preparing linear low density PE. The electron donor (ED) ispreferably an ether, like tetrahydrofuran. The catalyst component, asdefined, is used in polymerisation reactions together with analkylaluminum compound and optionally with an external electron donor.The optional external electron donor is said to be equal to or differentfrom the ED used in catalyst component. It can also be selected fromsilicon compounds of formula R_(a)R_(b)Si(OR)_(c), especiallycyclohexyltrimethoxysilane, t-butyltrimethoxysilane andthexyltrimethoxysilane. The polymerization process of WO2004055065comprises an optional pre-polymerisation step followed by a gas phasepolymerization step.

CN103304869 discloses a multimodal PE composition for pipes havingdensity of 0.935 to 0.945 g/cm³ and comprising three components 1)ethylene homopolymer (40-60 wt-%) with density more than 0.970 g/cm³ andmelt flow rate 5 (MFR₅) of more than 300 g/10 minutes, 2)ethylene-α-olefin copolymer (30-40 wt-%) with density of not greaterthan 0.935 g/cm³ and MFR₅ of not greater than 1 g/10 minutes and 3)ethylene-α-olefin copolymer (5-30 wt-%) with density less than 0.935g/cm³ and MFR₅ of not greater than 0.01 g/10 minutes. Each component hasnarrow molecular weight distribution (Mw/Mn) of less than or equal to 5and comonomer content of 0.2-0.7 mol-%. This composition is prepared inthe presence of a Ziegler-Natta catalyst and dimethoxydiphenlysilane orcyclohexyldimethoxysilane as external donor in a multistage processcomprising only slurry reactors. No information of catalyst productivityis given.

WO2013/113797 discloses similar type of multimodal PE composition asCN103304869 above having a low molecular weight ethylene polymercomponent and two higher molecular weight ethylene copolymer components.Polymer is produced in slurry polymerisation reactors, although otherreactor types are also generally mentioned. However, no external donorsare used.

WO2014102813 discloses a heterogeneous Ziegler-Natta catalyst systemcomprising a titanium procatalyst with a magnesium compound as a base,and at least one cocatalyst comprising at least one organoaluminiumcompound, a hydrocarbon medium and at least one external donorcomprising at least one organosilane compound. The catalyst system isobtained by adding said organoaluminium compound and organosilanecompound to the procatalyst system. The catalyst system is used forproducing UHMWPE (ultrahigh molecular weight polyethylene). Thepolymerisation process is a one-step polymerisation.

WO2009/027270 discloses a catalyst for ethylene polymerisationcomprising a solid catalyst component comprising titanium, magnesium andhalogen, an aluminum alkyl cocatalyst and a silane compound. Narrowmolecular weight distribution is desired indicated by FRR21/2 ratio atmost 30. Use of the catalyst for producing multimodal polymer or use ina multistage process is not discussed.

Although much development work in Ziegler-Natta catalyst has been donethere is still room for improvement. If specific polymer properties orspecific polymerisation processes or combinations thereof are desired,catalysts of prior art do not serve as appropriate catalysts as such,but modifications and adjustments are needed in order to get polymerwith desired properties and to produce said polymers with goodpolymerization productivity.

One method to allow the production of multimodal ethylene (co)polymerswith high molecular weight fraction and broad molecular weightdistribution (MWD) in a multistage process is to reduce or exclude theintroduction of hydrogen as a molecular weight controlling agent to atleast one of the polymerisation stages or reactors. However, if therelatively low molecular weight (co)polymer is produced in the stagebefore the stage, where the relatively high molecular weight copolymeris produced, it results, due to substantial hydrogen carry-over, in arelatively high hydrogen concentration in the reactor, where therelatively high molecular weight copolymer should be produced. Moreover,to provide polymer with good processability and improved flowproperties, multimodal polymers with a smaller proportion of the highmolecular weight fraction are often desired. However, this in turnresults easily in a relatively low ethylene concentration/partialpressure and therefore higher H₂/C₂ molar ratios, in the reactor, wherethe relatively high molecular weight copolymer is to be produced. Ifethylene (co)polymers with high molecular weight fraction are desired,and the amount of hydrogen has already been minimized, then externaladditives are added to the first polymerization stage. However, in thatcase, the problem is that polymers are often produced at the expense ofthe catalyst productivity. Further, in producing polyethylene in amultistage process comprising at least two stages one problem that isoften encountered with the prior art ZN-catalysts is that it isdifficult to produce an ethylene homo- or copolymer having broadmolecular weight distribution (MWD) (i.e. having melt flow rate ratioFRR_(21/5)≥40 and/or polydispersity index PDI≥27) and at the same timekeep productivity at a high level. I.e. in a beneficial process all thedesired beneficial polymer properties should not be obtained at theexpense of the overall catalyst productivity.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a process for producingethylene copolymer with desired properties. The present inventionprovides a multistage process for preparing a multimodal ethylenecopolymer with high molecular weight and broad molecular weightdistribution. It provides a process for producing multimodalpolyethylene in a process comprising at least two polymerisation stages,where at least one stage is carried out in a slurry phase and at leastone stage is carried out in gas phase in the presence of a Ziegler-Nattacatalyst comprising an external additive. Further, the inventionprovides a process, where the molecular weight of the polymer producedin the second stage can be increased without negatively affecting thecatalyst productivity. This is possible by overcoming limitations inhydrogen response of the catalyst and/or hydrogen carry-over from thereactor in which the relatively low molecular weight (co)polymer isproduced.

Further, the present invention provides a Ziegler-Natta catalystcomprising a solid catalyst component, a cocatalyst and an externaladditive as defined later in the present specification and whichcatalyst is suitable for producing ethylene polymers with desiredproperties in a multistage polymerisation process comprising at leastone stage carried out in a slurry phase and at least one stage carriedout in gas phase.

Further, one object of the invention is to use the catalyst inaccordance with the present invention in the process for producingpolyethylene, especially for producing ethylene copolymer in amultistage process, especially in a multistage process comprising atleast one slurry phase reactor and at least one gas phase reactor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a multistage process comprising at leastone slurry phase polymerization stage and at least one gas phasepolymerization stage for producing ethylene copolymers comprising thesteps of

-   -   (a1) introducing ethylene, optionally hydrogen and optionally        alpha-olefin comonomer having from 4 to 10 carbon atoms into an        optional polymerisation stage A1 in the presence of a solid        catalyst Ziegler-Natta component, a cocatalyst and optionally an        external additive,    -   (b1) maintaining said polymerisation stage in such conditions as        to produce an ethylene homo- or copolymer product P-A1    -   (a2-i) feeding ethylene, the polymerisation product P-A1,        optionally alpha-olefin comonomer having from 4 to 10 carbon        atoms and optionally an external additive to a polymerisation        stage A2, or    -   (a2-ii) feeding ethylene, a solid catalyst Ziegler-Natta        component, a cocatalyst, optionally alpha-olefin comonomer        having from 4 to 10 carbon atoms and optionally an external        additive to a polymerisation stage A2    -   (b2) maintaining said polymerisation stage A2 in such conditions        as to produce a lower molecular weight (co)polymer P-A2 or a        (co)polymer mixture P-M1 comprising the optional ethylene        (co)polymer P-A1 and the lower molecular weight ethylene        (co)polymer P-A2,    -   (c) feeding the polymerisation product P-A2 or the (co)polymer        mixture P-M1, additional ethylene and an alpha-olefin comonomer        having from 4 to 10 carbon atoms, an external additive, which        can be the same or different as the optional external additive        in step (a1) or (a2), optionally hydrogen and optionally        additional cocatalyst to the polymerisation stage B    -   (d) maintaining said polymerisation stage B in such conditions        as to produce a higher molecular weight polymerisation product        P-B,    -   (e) recovering the polymerisation product P-B from the        polymerisation stage B,        wherein the external additive has formula (I)        R¹ _(n)Si(OR²)_(4-n),  (I)        where n is an integer 0 to 3,        each R¹ are equal or different and are selected among hydrogen,        halogen, alkyl groups of 1 to 6 carbon atoms optionally        substituted with one or more halogen atoms, alkenyl groups of 2        to 6 carbon atoms optionally substituted with one or more        halogen atoms, and aryl groups of 6 to 12 carbon atoms        optionally substituted with one or more halogen atoms, or the R¹        groups can form with the Si atom they are linked to a ring of 3        to 8 ring atoms, provided that all R¹ are not hydrogen,        R² are equal or different and are selected among alkyl groups of        1 to 6 carbon atoms optionally substituted with one or more        halogen atoms, alkenyl groups of 2 to 6 carbon atoms optionally        substituted with one or more halogen atoms, and aryl groups of 6        to 12 carbon atoms optionally substituted with one or more        halogen atoms, or the OR² groups can form with the Si atom they        are linked to a ring of 3 to 8 ring atoms,        halogen is Br, Cl or F,        and wherein the polymerization stage B is a gas phase        polymerization stage.

The final polymer has preferably the melt flow rate ratio FRR_(21/5) ofat least 40 and/or polydispersity index PDI of at least 27.

Thus, the present invention provides a process for producing ethylenecopolymers having melt flow rate ratio FRR_(21/5) of at least 40 and/orPDI of at least 27 according to steps a) to e) as disclosed above.

More preferably the ethylene copolymers produced according to theprocess of the invention have melt flow rate ratio FRR_(21/5) of atleast 40 and PDI of at least 27.

Further, the present invention provides a Ziegler-Natta catalyst (C)comprising

i-1) a solid supported Ziegler-Natta catalyst component comprising acompound of Group 4 to 6 metal, optionally an aluminium compound,optionally an internal organic compound and a magnesium compoundsupported on an inorganic oxide support or

i-2) a solid supported Ziegler-Natta catalyst component comprising acompound of Group 4 to 6 metal, optionally an aluminium compound andoptionally an internal compound supported on a MgCl₂ based support,

ii) a cocatalyst of Group 13 metal compound and

iii) an external additive of formula (I)R¹ _(n)Si(OR²)_(4-n),  (I)where n is an integer 0 to 3,each R¹ are equal or different and are selected among H, halogen, alkylgroups of 1 to 6 carbon atoms optionally substituted with one or morehalogen atoms, alkenyl groups of 2 to 6 carbon atoms optionallysubstituted with one or more halogen atoms, and aryl groups of 6 to 12carbon atoms optionally substituted with one or more halogen atoms, orthe R¹ groups can form with the Si atom they are linked to a ring of 3to 8 ring atoms, provided that all R¹ are not hydrogen,R² are equal or different and are selected among alkyl groups of 1 to 6carbon atoms optionally substituted with one or more halogen atoms,alkenyl groups of 2 to 6 carbon atoms optionally substituted with one ormore halogen atoms, and aryl groups of 6 to 12 carbon atoms optionallysubstituted with one or more halogen atoms, or the OR² groups can formwith the Si atom they are linked to a ring of 3 to 8 ring atoms, andhalogen is Br, Cl or F.

The present invention relates also to the use of Ziegler-Natta catalyst(C) as defined above in a multistage process comprising at least oneslurry phase polymerization stage and at least one gas phasepolymerisation stage for producing ethylene copolymers, preferably forproducing ethylene copolymers having the melt flow rate ratio FRR_(21/5)of at least 40 and/or PDI of at least 27.

Preferred embodiments of the invention are described in dependent claimsas well in the following description.

According to the process of the invention ethylene copolymers areproduced by copolymerising ethylene monomers with one or morealpha-olefin comonomer units. The alpha-olefin comonomer units ofpolyethylene resins are selected from C₃-C₂₀-alpha-olefins, preferablyare selected from C₄-C₁₀-alpha-olefins, such as 1-butene, isobutene,1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene, 1-octene, 1-noneneand 1-decene, as well as dienes, such as butadiene, 1,7-octadiene and1,4-hexadiene, or cyclic olefins, such as norbornene, and any mixturesthereof. Most preferably, the comonomer is 1-butene and/or 1-hexene.

The inventors have now found that by using supported Ziegler-Nattacatalyst component and cocatalyst together with a specific externaladditive of alkoxysilane type, it is possible to broaden the preparationwindow of polyethylene. Especially it was found that the invention makesit possible to produce multimodal polyethylene where the molecularweight distribution (MWD) is increased in a multistage processcomprising at least one slurry phase polymerization stage carried out inat least one slurry phase reactor and at least one gas phasepolymerisation stage carried out in at least one gas phase reactor. Onespecific finding of the invention was that the molecular weight of thehigh molecular weight (MW) part of the multimodal ethylene copolymer canbe increased, i.e. the molecular weight of the final polymer isincreased.

The improvements, like the increase in molecular weight and increase ofMWD and/or PDI are not made at the expense of the productivity of thecatalyst, but the productivity remains still at high level, or is evenincreased.

The benefits of the invention are especially seen in the multistagepolymerisation process comprising at least two polymerization stages,and more specifically in a multistage process, where at least one stageis carried out in slurry phase and at least one stage in gas phase andwherein the catalyst is a Ziegler-Natta catalyst comprising a specificexternal additive.

Process

The general process configuration is described below.

In the present invention multimodal polymers with respect to themolecular weight distribution (MWD) are produced in a multistageprocess, where lower molecular weight and higher molecular weightpolymers (components) are produced at different polymerization stages.

The process of the present invention comprises at least twopolymerisation stages. Thus, the process of the present invention maycomprise three or more polymerisation stages.

Even though the present invention relates to a process for producingethylene polymer compositions in at least two polymerisation stages,especially in two or three stages, it should be understood that theprocess may contain additional polymerization stages in addition to theat least two stages disclosed above. It may contain as an additionalpolymerization stage e.g. a prepolymerization stage, as long as thepolymer produced in such additional stages does not substantiallyinfluence the properties of the polymer. Furthermore, any one of the atleast two polymerization stages disclosed above may be conducted as twoor more sub-stages, provided that the polymer produced in each suchsub-stage as well as their mixture matches the description for thepolymer for the respective stage.

However, it is preferred to conduct each of the polymerization stage asa single polymerization stage each carried out in one polymerisationreactor in order to prevent the process from becoming unnecessarilycomplex. Therefore, in the most preferred embodiment the processconsists of at least two polymerization stages, each stage carried outin one reactor, and which may be preceded by a prepolymerization stage.

The term multimodal copolymer describes in general a copolymer whichcontains distinct components having different average molecular weightsor different contents of comonomer or both. The multimodal copolymer isproduced by copolymerizing ethylene and a comonomer in two or morepolymerization stages where the polymerization conditions aresufficiently different to allow production of different polymers indifferent stages. In the present invention an essential feature is thatthe final polymer is multimodal in respect of molecular weight.

Preferably the process is a continuously operated process.

The term, continuously operating process, describes a process or aprocess stage into which the feedstock materials are continuously orintermittently introduced and from which the product is continuously orintermittently withdrawn. By continuous addition or withdrawal is meantthat an uninterrupted stream goes in or flows out of the process orprocess stage. By intermittent addition or withdrawal is meant thatduring the operation of the process small batches of raw material areconstantly added into or product is constantly withdrawn from theprocess or process stage. The cycle time between such batches is smallcompared to the overall average residence time of the process or processstage, such as not more than 10% of the overall average residence time.

According to the preferred embodiment the polymerization process of thepresent invention is conducted in a cascaded sequence comprising one ortwo slurry phase polymerisation reactors, more preferably two loopreactors, followed by a gas phase reactor.

The slurry polymerization may be conducted in any known reactor used forslurry polymerization. Such reactors include a continuous stirred tankreactor and a loop reactor. It is especially preferred to conduct theslurry polymerization in loop reactor(s).

Pre-Polymerization Stage

The polymerization steps may be preceded by a pre-polymerization step.The purpose of the pre-polymerisation is to polymerize a small amount ofpolymer onto the catalyst at a relatively low temperature. Bypre-polymerisation it is possible to substantially improve theperformance of the catalyst in the following stages. Thepre-polymerisation step may be conducted in slurry or in gas phase.Preferably pre-polymerization is conducted in slurry.

Thus, the pre-polymerisation step may be conducted in a loop reactor.The pre-polymerisation is then preferably conducted in an inert diluent,typically a hydrocarbon diluent such as methane, ethane, propane,n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or theirmixtures. Preferably the diluent is a low-boiling hydrocarbon havingfrom 1 to 4 carbon atoms or a mixture of such hydrocarbons.

The temperature in the pre-polymerisation step is typically from 0 to90° C., preferably from 20 to 80° C. and more preferably from 30 to 70°C.

The pressure is not critical and is typically from 1 to 150 bar,preferably from 40 to 80 bar.

The amount of monomer is typically such that from about 0.1 to 15000grams of monomer per one gram of solid catalyst component, preferably 50to 5000 grams of monomer per one gram of solid catalyst component, ispolymerized in the pre-polymerisation step. As the person skilled in theart knows, the catalyst particles recovered from a continuouspre-polymerization reactor do not all contain the same amount ofprepolymer. Instead, each particle has its own characteristic amountwhich depends on the residence time of that particle in thepre-polymerization reactor. As some particles remain in the reactor fora relatively long time and some for a relatively short time, then alsothe amount of prepolymer on different particles is different and someindividual particles may contain an amount of prepolymer which isoutside the above limits. However, the average amount of prepolymer onthe catalyst typically is within the limits specified above.

In addition to ethylene monomer it is possible to use one or morealpha-olefin comonomers in the pre-polymerisation step to reducecrystallinity of pre-polymer and/or to increase catalyst activity ifdesired. Suitable comonomers are, for example, propene, 1-butene,1-hexene, 4-methyl-1-pentene, 1-octene and their mixtures.

The molecular weight and crystallinity of the pre-polymer may becontrolled also by hydrogen as it is known in the art. Further,antistatic additive may be used to prevent the particles from adheringto each other or the walls of the reactor, as disclosed in WO-A-96/19503and WO-A-96/32420.

The solid catalyst component and cocatalyst are preferably allintroduced to the pre-polymerisation step together. However, the solidcatalyst component and the cocatalyst can be fed separately. Moreover,it is possible that only a part of the cocatalyst is introduced into thepre-polymerisation stage and the remaining part into subsequentpolymerization stages.

The catalyst may be transferred into the (pre)polymerization reactor byany means known in the art. It is thus possible to suspend the catalystin a diluent and maintain it as homogeneous slurry. Especially preferredit is to use oil having a viscosity from 20 to 1500 mPa·s as diluent, asdisclosed in WO-A-2006/063771. It is also possible to mix the catalystwith a viscous mixture of grease and oil and feed the resultant pasteinto the polymerization zone. Further still, it is possible to let thecatalyst settle and introduce portions of thus obtained catalyst mudinto the polymerization zone in a manner disclosed, for instance, inEP-A-428054.

Optional Polymerization Stage A1

In the optional polymerization stage A1, an ethylene (co)polymer can beproduced. This is done by introducing a polymerization catalyst,optionally through the prepolymerization stage as described above, intothe polymerization stage A1 together with ethylene, optionallycomonomer, optionally hydrogen and optionally an external additive toproduce ethylene (co)polymer P-A1.

The ethylene (co)polymer P-A1 has a melt flow rate MFR₂ of from 0 to1000 g/10 min, preferably from 0 to 750 g/10 min The ethylene(co)polymer P-A1 has Mw from 15000 to 5000000, preferably from 20000 to3500000.

The optional polymerization stage A1 is conducted as a particle formingprocess. In such a process the polymerization catalyst is introducedinto the process in particle form, preferably through thepre-polymerization step as described above. The first ethylene(co)polymer then grows on the catalyst particles thereby forming amixture of a fluid reaction mixture and the particles comprising thefirst polymer.

The polymerization stage A1 is preferably conducted as a slurrypolymerization. The slurry polymerization usually takes place in aninert diluent, typically a hydrocarbon diluent such as methane, ethane,propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc.,or their mixtures. Preferably the diluent is a low-boiling hydrocarbonhaving from 1 to 4 carbon atoms or a mixture of such hydrocarbons. Anespecially preferred diluent is propane, possibly containing minoramount of methane, ethane and/or butane.

The ethylene content in the fluid phase of the slurry may be from 1 toabout 50% by mole, preferably from about 2 to about 20% by mole and inparticular from about 2 to about 10% by mole. The benefit of having ahigh ethylene concentration is that the productivity of the catalyst isincreased but the drawback is that more ethylene then needs to berecycled than if the concentration was lower.

The temperature in the optional polymerization stage A1 is typicallyfrom 30 to 100° C., preferably from 40 to 95° C. An excessively hightemperature should be avoided to prevent partial dissolution of thepolymer into the diluent and the fouling of the reactor. The pressure isfrom 1 to 150 bar, preferably from 40 to 80 bar.

The slurry polymerization may be conducted in any known reactor used forslurry polymerization. Such reactors include a continuous stirred tankreactor and a loop reactor. It is especially preferred to conduct thepolymerization in loop reactor. In such reactors the slurry iscirculated with a high velocity along a closed pipe by using acirculation pump. Loop reactors are generally known in the art andexamples are given, for instance, in U.S. Pat. Nos. 4,582,816,3,405,109, 3,324,093, EP-A-479186 and U.S. Pat. No. 5,391,654.

The slurry may be withdrawn from the reactor either continuously orintermittently. A preferred way of intermittent withdrawal is the use ofsettling legs where slurry is allowed to concentrate before withdrawinga batch of the concentrated slurry from the reactor. The use of settlinglegs is disclosed, among others, in U.S. Pat. Nos. 3,374,211, 3,242,150and EP-A-1310295. Continuous withdrawal is disclosed, among others, inEP-A-891990, EP-A-1415999, EP-A-1591460 and WO-A-2007/025640. Thecontinuous withdrawal is advantageously combined with a suitableconcentration method, as disclosed in EP-A-1310295 and EP-A-1591460. Itis preferred to withdraw the slurry from the polymerization stage A1continuously.

Hydrogen is optionally introduced into the polymerization stage A1 forcontrolling the MFR₂ of the first copolymer. The amount of hydrogenneeded to reach the desired MFR depends on the catalyst used and thepolymerization conditions.

The average residence time in the polymerization stage A1 is typicallyfrom 20 to 120 minutes, preferably from 30 to 80 minutes. As it is wellknown in the art the average residence time τ can be calculated from:

$\begin{matrix}{\tau = \frac{V_{R}}{Q_{o}}} & ( {{eq}.\mspace{14mu} 1} )\end{matrix}$

Where V_(R) is the volume of the reaction space (in case of a loopreactor, the volume of the reactor, in case of the fluidized bedreactor, the volume of the fluidized bed) and Q_(o) is the volumetricflow rate of the product stream (including the polymer product and thefluid reaction mixture).

The production rate in the polymerization stage A1 is suitablycontrolled with the catalyst feed rate. It is also possible to influencethe production rate by suitable selection of the monomer concentrationin the polymerization stage A1. The desired monomer concentration canthen be achieved by suitably adjusting the ethylene feed rate into thepolymerization stage A1.

Polymerization Stage A2

The polymerisation stage A2 is carried out in the first slurry reactor(if only one slurry phase reactor is used) or in a second slurry reactorof the polymerisation configuration (if two slurry phase reactors areused). Stage A2 is followed by polymerization stage B carried out in agas phase reactor.

In the polymerization stage A2 a (co)polymer P-A2 or (co)polymer mixtureP-M1 comprising the optional ethylene (co)polymer P-A1 (produced in theoptional stage A1) and a lower molecular weight (Mw) ethylene(co)polymer P-A2 is formed. This is done by introducing active catalysttogether with ethylene or feeding ethylene and the particles of thepolymer P-A1 containing active catalyst dispersed therein into thepolymerization stage A2. Hydrogen and optionally an alpha-olefincomonomer are introduced for controlling the molecular weight anddensity, respectively, as described above for the optionalpolymerization stage A1. An external additive is optionally fed to thepolymerisation stage A2.

The melt flow rate MFR₂ of the (co)polymer P-A2 or (co)polymer mixtureP-M1 is from 0 to 1000 g/10 min, preferably from 0.1 to 750 g/10 min andmore preferably from 0.2 to 600 g/10 min. Furthermore, the density ofthe (co)polymer P-A2 or (co)polymer mixture P-M1 is from 935 to 975kg/m³, preferably from 940 to 975 kg/m³ and most preferably from 945 to975 kg/m³.

The polymerization in the polymerization stage A2 is advantageouslyconducted as a slurry polymerization as described above for the optionalpolymerization stage A1. The temperature in the polymerization stage A2is suitably from 60 to 100° C., preferably from 65 to 95° C. Thepressure is suitably from 1 to 150 bar, preferably from 40 to 80 bar.The polymerization stage A2 is conducted in one or more loop reactors,preferably in one loop reactor.

Hydrogen feed is adjusted to achieve a desired melt flow rate (ormolecular weight) of the (co)polymer P-A2 or (co)polymer mixture P-M1.Suitably, the hydrogen feed is controlled to maintain constant hydrogento ethylene molar ratio in the reaction mixture. The actual ratiodepends on the catalyst as well as the type of the polymerization. Thedesired polymer properties have been obtained in slurry polymerizationin a loop reactor by maintaining the H₂/ethylene ratio within the rangeof from 200 to 1000 mol/kmol, preferably from 200 to 800 mol/kmol.

The optional alpha-olefin comonomer is introduced into thepolymerization stage A2 for controlling the density of the (co)polymerP-A2 or (co)polymer mixture P-M1. The amount of the comonomer needed toreach the desired density depends on the comonomer type, the catalystused and the polymerization conditions.

The desired polymer production rate in the polymerization stage A2 maybe reached by suitably selecting the ethylene concentration in saidpolymerization stage A2, in the same way as was described above for theoptional polymerization stage A1.

The average residence time in the polymerization stage A2 is typicallyfrom 20 to 120 minutes, preferably from 30 to 80 minutes.

If the polymerisation stage A1 is used, the (co)polymer mixture P-M1comprises from 10 to 60% by weight of the first polymer P-A1 and from 40to 90% by weight of the (co)polymer P-A2. Preferably, the (co)polymermixture P-M1 comprises from 20 to 55% by weight of the first polymer andfrom 45 to 80% by weight of the (co)polymer P-A2. In case polymerisationstage A1 is not used, the polymer mixture P-M1 constitutes the polymerP-A2 only. This is another preferred alternative.

Typically at least a part of the fluid reaction mixture present in thepolymerization stage A2 is removed from the polymer. This makes itpossible to have a sufficient difference between the molecular weightsof the polymers produced in the polymerization stage A2 and thesubsequent polymerization stage B.

The (co)polymer P-A2 or (co)polymer mixture P-M1 is then directed to thepolymerization stage B whereas the fluid reaction mixture may bedirected to a recovery section or alternatively, the removed fluidreaction mixture may be returned wholly or partly into thepolymerization stage A1 or A2. In the recovery section the components ofthe reaction mixture are separated to produce, for instance, recoveredstreams of monomers and diluent. The recovered streams may then bereused in the polymerization process. The removal of the fluid reactionmixture from the polymer may be done by any means known in the art, suchas by flashing or extracting. Flashing is usually preferred because itis a simple and effective process. For instance EP-A-1415999 discloses asuitable method for transferring the polymer from the previous stage tothe next polymerization stage.

According to a preferred embodiment the process of the inventioncomprises two slurry reactors, more preferably two loop reactors (stageA1 and stage A2).

Polymerization Stage B

The polymerisation stage B is carried out in the gas phase reactor ofthe polymerisation configuration, where one or two slurry reactorsfollowed by a gas phase reactor are used. In case only one slurryreactor is used, stage B is the second polymerization stage and in casetwo slurry reactors are used B is the third polymerization stage.

In the polymerization stage B a higher molecular weight copolymer P-B isformed. P-B is the final (co)polymer mixture comprising (co)polymer fromstage A2, (i.e. P-A2 or the (co)polymer mixture P-M1) and a copolymer ofethylene from stage B. This is done by introducing the particles of the(co)polymer from stage A2 (P-A2 or (co)polymer mixture P-M1), containingactive catalyst dispersed therein, together with additional ethylene andan alpha-olefin comonomer into the polymerization stage B and continuingthe polymerisation under polymerisation conditions as defined below.This causes the copolymer P-B to form on the particles containing thepolymer product of stage A2.

Hydrogen may be introduced for controlling the molecular weight. Thedesired polymer properties have been obtained in gas phasepolymerization in a fluidized bed reactor by maintaining the molar ratioof hydrogen to ethylene within the range of from 1 to 200 mol/kmol,preferably from 1 to 150 mol/kmol.

According to the process of the present invention external additive isfed to the polymerization stage B.

The alpha-olefin comonomer is typically introduced to maintain aconstant comonomer to ethylene ratio in the reaction mixture. Thecomonomer is an alpha-olefin having from 4 to 10 carbon atoms and may bethe same as the optional first alpha-olefin comonomer or it may bedifferent therefrom. Preferably the alpha-olefin comonomer is 1-butene,1-hexene or 1-octene, more preferably 1-butene or 1-hexene. In case acomonomer is introduced into the previous stage, the comonomer isinevitably carried over from the previous polymerization stage into thethird polymerization stage. The comonomer to ethylene ratio that isneeded to produce a polymer with the desired density depends, amongothers, on the type of comonomer and the type of catalyst. With 1-hexeneas a comonomer the desired polymer properties have been obtained in gasphase polymerization in a fluidized bed reactor with a molar ratio of1-hexene to ethylene from 1 to 200 mol/kmol, preferably from 5 to 100mol/kmol.

The polymerization in gas phase may be conducted in a fluidized bedreactor, in a fast fluidized bed reactor or in a settled bed reactor orin any combination of these. When a combination of reactors is used thenthe polymer is transferred from one polymerization reactor to another.Furthermore, a part or whole of the polymer from a polymerization stagemay be returned into a prior polymerization stage.

The desired polymer production rate in the polymerization stage B may bereached by suitably selecting the ethylene concentration in saidpolymerization stage, in the same way as was described above for theslurry polymerization stages.

Preferably the polymerization stage B is conducted as a fluidized bedgas phase polymerization. In a fluidized bed gas phase reactor an olefinis polymerized in the presence of a polymerization catalyst in anupwards moving gas stream. The reactor typically contains a fluidizedbed comprising the growing polymer particles containing the activecatalyst located above a fluidization grid.

The polymer bed is fluidized with the help of the fluidization gascomprising the olefin monomer, optional comonomer(s), optional chaingrowth controllers or chain transfer agents, such as hydrogen, andoptional inert gas. The fluidization gas is introduced into an inletchamber at the bottom of the reactor. To make sure that the gas flow isuniformly distributed over the cross-sectional surface area of the inletchamber the inlet pipe may be equipped with a flow dividing element asknown in the art, e.g. U.S. Pat. No. 4,933,149 and EP-A-684871. One ormore of the above-mentioned components may be continuously added intothe fluidization gas to compensate for losses caused, among other, byreaction or product withdrawal.

From the inlet chamber the gas flow is passed upwards through afluidization grid into the fluidized bed. The purpose of thefluidization grid is to divide the gas flow evenly through thecross-sectional area of the bed. Sometimes the fluidization grid may bearranged to establish a gas stream to sweep along the reactor walls, asdisclosed in WO-A-2005/087361. Other types of fluidization grids aredisclosed, among others, in U.S. Pat. No. 4,578,879, EP 600414 andEP-A-721798.

The fluidization gas passes through the fluidized bed. The superficialvelocity of the fluidization gas must be higher than the minimumfluidization velocity of the particles contained in the fluidized bed,as otherwise no fluidization would occur. On the other hand, thevelocity of the gas should be lower than the onset velocity of pneumatictransport, as otherwise the whole bed would be entrained with thefluidization gas. The minimum fluidization velocity and the onsetvelocity of pneumatic transport can be calculated when the particlecharacteristics are known by using common engineering practise.

When the fluidization gas is contacted with the bed containing theactive catalyst the reactive components of the gas, such as monomers,comonomers and hydrogen, react in the presence of the catalyst toproduce the polymer product. At the same time the gas is heated by thereaction heat.

The unreacted fluidization gas is removed from the top of the reactorand cooled in a heat exchanger to remove the heat of reaction. The gasis cooled to a temperature which is lower than that of the bed toprevent the bed from heating because of the reaction. It is possible tocool the gas to a temperature where a part of it condenses. When theliquid droplets enter the reaction zone they are vaporised. Thevaporisation heat then contributes to the removal of the reaction heat.This kind of operation is called condensed mode and variations of it aredisclosed, among others, in WO-A-2007/025640, U.S. Pat. No. 4,543,399,EP-A-699213 and WO-A-94/25495. It is also possible to add condensingagents into the recycle gas stream, as disclosed in EP-A-696293. Thecondensing agents are non-polymerizable components, such as n-pentane,isopentane, n-butane or isobutane, which are at least partiallycondensed in the cooler.

The gas is then compressed and recycled into the inlet chamber of thereactor. Prior to the entry into the reactor fresh reactants areintroduced into the fluidization gas stream to compensate for the lossescaused by the reaction and product withdrawal. It is generally known toanalyze the composition of the fluidization gas and introduce the gascomponents to keep the composition constant. The actual composition isdetermined by the desired properties of the product and the catalystused in the polymerization.

The catalyst may be introduced into the reactor in various ways, eithercontinuously or intermittently. Among others, WO-A-01/05845 andEP-A-499759 disclose such methods. Where the gas phase reactor is a partof a reactor cascade the catalyst is usually dispersed within thepolymer particles from the preceding polymerization stage. The polymerparticles may be introduced into the gas phase reactor as disclosed inEP-A-1415999 and WO-A-00/26258.

The polymeric product may be withdrawn from the gas phase reactor eithercontinuously or intermittently. Combinations of these methods may alsobe used. Continuous withdrawal is disclosed, among others, inWO-A-00/29452. Intermittent withdrawal is disclosed, among others, inU.S. Pat. No. 4,621,952, EP-A-188125, EP-A-250169 and EP-A-579426.

The top part of the gas phase reactor may include a so calleddisengagement zone. In such a zone the diameter of the reactor isincreased to reduce the gas velocity and allow the particles that arecarried from the bed with the fluidization gas to settle back to thebed.

The bed level may be observed by different techniques known in the art.For instance, the pressure difference between the bottom of the reactorand a specific height of the bed may be recorded over the whole lengthof the reactor and the bed level may be calculated based on the pressuredifference values. Such a calculation yields a time-averaged level. Itis also possible to use ultrasonic sensors or radioactive sensors. Withthese methods instantaneous levels may be obtained, which of course maythen be averaged over time to obtain a time-averaged bed level.

Also antistatic agent(s) may be introduced into the gas phase reactor ifneeded. Suitable antistatic agents and methods to use them aredisclosed, among others, in U.S. Pat. Nos. 5,026,795, 4,803,251,4,532,311, 4,855,370 and EP-A-560035. They are usually polar compoundsand include, among others, water, ketones, aldehydes and alcohols.

The reactor may also include a mechanical agitator to further facilitatemixing within the fluidized bed. An example of suitable agitator designis given in EP-A-707513.

Typically the fluidized bed polymerization reactor is operated at atemperature within the range of from 50 to 100° C., preferably from 65to 90° C. The pressure is suitably from 10 to 40 bar, preferably from 15to 30 bar.

The average residence time in the polymerization stage B is typicallyfrom 40 to 240 minutes, preferably from 60 to 180 minutes.

The polymerization stage B is conducted in one or more gas phasereactors, more preferably in one fluidized bed reactor.

The final copolymer mixture (P-B) typically comprises from 35 to 70% byweight of the (co)polymer from stage A2 ((co)polymer P-A2 or (co)polymermixture P-M1) and from 30 to 65% by weight of the copolymer produced instage B.

Suitable processes comprising cascaded slurry and gas phasepolymerization stages are disclosed, among others, in WO-A-92/12182 andWO-A-96/18662 of Borealis and known as Borstar technology.

The external additive is fed to the actual polymerisation stage. Theessential feature of the multistage process of the present inventioncomprising at least one slurry phase reactor and at least one gas phasereactor is that the external additive is fed to the gas phasepolymerization stage B. The external additive is optionally fed to theoptional polymerization stage A1 and/or to the stage A2, however,preferably the external additive is fed to the gas phase stage B only.

Catalysts

The solid catalyst component of the catalyst of the present invention istypically a supported Ziegler-Natta catalyst component. Suitablecatalyst components are disclosed in patents as listed above anddescribed below. The solid catalyst component might also containinternal organic compounds or internal electron donors as known in theart.

In this specification, the internal organic compound or internalelectron donor is a compound being part of the solid catalyst componentand added into said solid catalyst component during its preparation. Theexternal additive is not part of the solid catalyst component but fed tothe polymerization process either separately or together with the solidcatalyst component or with the cocatalyst as defined in the presentdisclosure.

Thus, the catalyst according to the invention, which is used in thepolymerization process according to the invention comprises i) a solidsupported Ziegler-Natta catalyst component ii) an organometalliccocatalyst and iii) a specific external additive.

The solid catalyst component i) used in the present invention comprisesat least a transition metal compound of Group 4 to 6 of the PeriodicTable (IUPAC, Nomenclature of Inorganic Chemistry, 1989), preferably acompound of Group 4 metal or a vanadium compound, most preferably atitanium compound, a metal compound of Group 1 to 3 of the PeriodicTable (IUPAC), most preferably magnesium compound, and, optionally, acompound of group 13 of the Periodic Table (IUPAC), most preferablyaluminium compound, and optionally an internal organic compound.

Thus, the catalyst component contains preferably a magnesium compound, atitanium compound, optionally an aluminium compound and optionally aninternal organic compound supported on a particulate support, or thecatalyst component comprises a titanium compound, optionally analuminium compound and optionally an internal organic compound supportedon a magnesium dihalide based support.

The magnesium compound is preferably a reaction product of an alcoholwith magnesium dialkyl, magnesium alkyl alkoxy or magnesium dialkoxy.More preferably magnesium dialkyl is used. The alcohol is a linear orbranched aliphatic mono-alcohol of 2 to 16 carbon atoms. Preferably, thealcohol has from 4 to 16 carbon atoms. Branched alcohols are especiallypreferred, and 2-ethyl-1-hexanol is one example of the preferredalcohols. The magnesium dialkyl may be any compound of magnesium bondingto two alkyl (or two alkoxy or one alkyl and one alkoxy) groups, whichmay be the same or different. Alkyl and alkoxy groups have typically 1to 18 carbon atoms, preferably 2 to 12 carbon atoms. Butyl-octylmagnesium is one example of the preferred magnesium dialkyls.

The aluminium compound is typically trialkyl aluminium or chlorinecontaining aluminium alkyl. Especially preferred compounds are aluminiumalkyl dichlorides, dilalkyl aluminium chloride and aluminium alkylsesquichlorides, or trialkylaluminium. Alkyl groups are preferablyalkyls with 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms.

The titanium compound is preferably a halogen containing titaniumcompound, preferably chlorine containing titanium compound. Especiallypreferred titanium compound is titanium tetrachloride.

A typical internal organic compound is chosen from the followingclasses: ethers, esters, amines, ketones, alcohols, anhydrides ornitriles or mixtures thereof. Preferably the internal organic compoundis selected from ethers and esters, most preferably from ethers.Preferred ethers are of 2 to 20 carbon-atoms and especially mono, di ormulticyclic saturated or unsaturated ethers comprising 3 to 6 ringatoms. Typical cyclic ethers suitable in the present invention, if used,are tetrahydrofuran (THF), substituted THF, like 2-methyl THF, di-cyclicethers, like 2,2-di(2-tetrahydrofuryl)propane, 2,2-di-(2-furan)-propane,or isomers or mixtures thereof. Internal organic compounds are alsooften called as internal electron donors.

The particulate support or carrier material can be an inorganic oxidesupport, such as silica, alumina, titania, silica-alumina andsilica-titania, typically silica.

The catalyst can be prepared by sequentially contacting the carrier withthe above mentioned compounds, as described e.g. in EP 688794 and WO99/51646. Alternatively, it can be prepared by first preparing asolution from the components and then contacting the solution with acarrier, as described in WO 01/55230.

Alternatively the catalyst component used in the present invention maybe supported on MgCl₂. Such catalysts are disclosed widely in prior art,for instance, in EP376936, WO 2005/118655 and EP 810235, or can be amodified versions thereof. According to one preferred modificationmethod the catalyst may be prepared by contacting spheroidal or granularMgCl₂*mROH, like MgCl₂*mEtOH, carrier material with an internal organiccompound, preferably with a dicyclic ether compound, in the beginning ofthe catalyst synthesis before a treatment with the titanium compound(e.g. TiCl₄) or even before treating the MgCl₂*mEtOH carrier materialwith a Group 13 compound and finally recovering the solid catalystcomponent.

Accordingly, one preferred catalyst as described above and used in thepresent invention comprises a solid MgCl₂ supported component which isprepared by a method comprising the steps:

-   -   a) providing solid carrier particles of MgCl₂*mROH adduct    -   b) pre-treating the solid carrier particles of step a) with a        compound of Group 13 metal    -   c) treating the pre-treated solid carried particles of step b)        with a transition metal compound of Group 4 to 6    -   d) recovering the solid catalyst component        wherein the solid carrier particles are contacted with an        internal organic compound of formula (II) or isomers or mixtures        therefrom before treating the solid carrier particles in step c)

andwherein in the formula (II) or isomers or mixtures therefromR₁ to R₅ are the same or different and can be hydrogen, a linear orbranched C₁ to C₈-alkyl group, or a C₃-C₈-alkylene group, or two or moreof R₁ to R₅ can form a ring,the two oxygen-containing rings are individually saturated or partiallyunsaturated or unsaturated, andR in the adduct MgCl₂*mROH is a linear or branched alkyl group with 1 to12 C atoms, and m is 0 to 6.

The cocatalysts ii), which are also known as activators, are organometalcompounds of Group 13 metal, typically organoaluminium compounds. Thesecompounds include alkyl aluminium compounds and alkyl aluminium halides.Typical trialkylaluminium compounds are trimethylaluminium,triethylaluminium, tri-isobutylaluminium, trihexylaluminium andtri-n-octylaluminium or other aluminium alkyl compounds, such asisoprenylaluminium, and typical alkyl aluminium halides include alkylaluminium chlorides, such as ethylaluminium dichloride, diethylaluminiumchloride, ethylaluminium sesquichloride, dimethylaluminium chloride andthe like. Especially preferred cocatalysts are trialkylaluminiums, ofwhich triethylaluminium, trimethylaluminium and tri-isobutylaluminiumare particularly used.

As indicated above, the essential feature of the present invention isthat a specific type of external additive iii) is used. The externaladditives used in the present invention are alkoxysilane type externaladditives. More specific the external additive has formula (I)R¹ _(n)Si(OR²)_(4-n),  (I)where n is an integer 0 to 3,each R¹ can be equal or different and are selected among H, halogen,alkyl groups of 1 to 6 carbon atoms and alkenyl groups of 2 to 6 carbonatoms both optionally substituted with one or more halogen atoms, andaryl groups of 6 to 12 carbon atoms optionally substituted with one ormore halogen atoms, or the R¹ groups can form with the Si atom they arelinked to a ring of 3 to 8 ring atoms, provided that all R¹ are nothydrogen,R² can be equal or different and are selected among alkyl groups of 1 to6 carbon atoms and alkenyl groups of 2 to 6 carbon atoms both optionallysubstituted with one or more halogen atoms, and aryl groups of 6 to 12carbon atoms optionally substituted with one or more halogen atoms, orthe OR² groups can form with the Si atom they are linked to a ring of 3to 8 ring atoms,andhalogen is Br, Cl or F.

Mixtures of alkoxysilanes of formula (I) are also within the scope ofthe present invention.

The external additive is used in polymerisation process in amountscorresponding Si/Ti mol/mol ratio of 0.2 to 5.0, preferably 0.3 to 3,more preferably 0.5 to 2.5.

In embodiments of formula (I)

n is preferably an integer 1 to 3,

all R¹ groups are preferably the same or different and are hydrogen oralkyl groups of 1 to 6 carbon atoms or aryl groups of 6 to 12 carbonatoms, more preferably all R¹ groups are the same or different and arehydrogen or linear alkyl groups of 1 to 3 carbon atoms, more preferablyalkyl groups of 1 to 2 carbon atoms, provided that all R¹ are nothydrogen,all R² groups are preferably the same and are alkyl groups of 1 to 3carbon atoms, more preferably alkyl groups of 1 to 2 carbon atoms.

Thus, in a most preferred embodiment of formula (I), each R¹ isindependently hydrogen or methyl or ethyl, provided that at least one R¹is methyl or ethyl, R² is methyl or ethyl and n is 1 or 2.

Preferred external additives used in the present invention aredimethoxydimethylsilane, trimethoxymethylsilane, diethoxydimethylsilane,dimethoxydiethylsilane, dimethoxydi-n-propylsilane,dimethoxy(methyl)silane, vinylmethyldimethoxysilane,chloromethyl(methyl)dimethoxysilane, dimethoxymethylphenylsilane,3-chloropropyldimethoxymethylsilane,trimethoxy(3,3,3-trifluoropropyl)silane, 3-chloropropyltrimethoxysilane.

Especially preferred alkoxysilanes are dimethoxy(methyl)silane,dimethoxydimethylsilane and trimethoxymethylsilane.

Thus, according to a preferred embodiment of the invention themultistage process for producing ethylene copolymers comprises the stepsof

-   -   (a1) introducing ethylene, optionally hydrogen and optionally        alpha-olefin comonomer having from 4 to 10 carbon atoms into an        optional polymerisation stage A1 in the presence of a solid        catalyst Ziegler-Natta component, a cocatalyst and optionally an        external additive,    -   (b1) maintaining said polymerisation stage in such conditions as        to produce an ethylene homo- or copolymer product P-A1    -   (a2-i) feeding ethylene, the polymerisation product P-A1,        optionally alpha-olefin comonomer having from 4 to 10 carbon        atoms and optionally an external additive to a polymerization        stage A2, or    -   (a2-ii) feeding ethylene, a solid catalyst Ziegler-Natta        component, a cocatalyst, optionally alpha-olefin comonomer        having from 4 to 10 carbon atoms and optionally an external        additive to a polymerization stage A2    -   (b2) maintaining said polymerisation stage A2 in such conditions        as to produce a lower Mw (co)polymer P-A2 or a (co)polymer        mixture P-M1 comprising the optional ethylene (co)polymer P-A1        and the lower Mw ethylene (co)polymer P-A2,    -   (c) feeding the polymerisation product P-A2 or the (co)polymer        mixture P-M1, additional ethylene and an alpha-olefin comonomer        having from 4 to 10 carbon atoms, an external additive, which        can be the same or different as the optional external additive        in step (a1) or (a2), optionally hydrogen and optionally        additional cocatalyst to the polymerization stage B,    -   (d) maintaining said polymerisation stage B in such conditions        as to produce a higher molecular weight polymerisation product        P-B,    -   (e) recovering the polymerisation product P-B from the        polymerization stage B,        wherein the external additive has the formula (I)        R¹ _(n)Si(OR²)_(4-n),  (I)        where        n is an integer 1 to 3,        all R¹ groups are the same or different and are hydrogen or        alkyl groups of 1 to 6 carbon atoms or aryl groups of 6 to 12        carbon atoms, more preferably all R¹ groups are the same or        different and are hydrogen or alkyl groups of 1 to 3 carbon        atoms, more preferably alkyl groups of 1 to 2 carbon atoms,        provided that all R¹ are not hydrogen,        all R² groups are the same and are alkyl groups of 1 to 3 carbon        atoms, more preferably alkyl groups of 1 to 2 carbon atoms,        and wherein the optional polymerization stage A1 and the        polymerization stage A2 are slurry polymerization stages and        polymerization stage B is a gas phase polymerization stage.

Slurry reactors are preferably loop reactors in all embodiments of theinvention. The polymerization stage B comprises at least one gas phasereactor, preferably one gas phase reactor.

In the preferred embodiment the external additive is added only to thegas phase reactor of the multistage process comprising slurry and gasphase reactors.

Polymer Properties

According to the process of the invention it's possible to produceethylene copolymers with very broad MWD in a multistage process, andstill keep the productivity on a good level (FIG. 2). MFR₅ values of thefinal polymer (P-B) from as low as 0.03 g/10 min are possible.Representative MFR₅ value ranges can be from 0.03 g/10 min to 5 g/10min, preferably from 0.05 to 3 g/10 min and more preferably from 0.07 to1 g/10 min. (190° C., 5 kg load).

The low MFR₅ together with high FFR_(21/5) values indicate that thepolymers produced in the gas-phase reactor have high molecular weight.

Furthermore, it is preferred that the melt flow rate ratio FRR_(21/5) ofthe final polymer is at least 40, preferably more than 50, e.g. at least52, more preferably at least 55. Thus, the flow rate ratio FRR_(21/5) ispreferably in the range of 40 to 100, preferably 50<FRR_(21/5)≤100, likein the range of 52 to 100, more preferably in the range of 55 to 90,indicating broad molecular weight distribution. In addition, inpreferred embodiment the polydispersity index PDI of the final polymeris at least 27.

The alpha-olefin comonomer used in the polymerization process of theinvention is selected from alpha-olefins containing 4 to 10 carbonatoms, most preferably from 1-butene and 1-hexene. The content of thecomonomer is controlled to obtain the desired density of the finalpolymer.

Typically the final polymer has a density of from 920 to 965 kg/m³,preferably from 935 to 960 kg/m³, more preferably from 940 to 957 kg/m³.

DESCRIPTION OF THE FIGURES

FIG. 1 shows the molecular weight curves of inventive example 4 (IE4)and of comparative examples 3 (CE3) indicating the differences inmodality of the polymers.

FIG. 2 shows the molecular weight curves of inventive example 6 (IE6)and of comparative examples 4 (CE4) indicating the differences inmodality of the polymers.

FIG. 3 shows the FRR_(21/5) ratio and productivity vs. Si/Ti mol/molratio of the comparative and inventive examples.

EXPERIMENTAL PART

Methods

Melt Flow Rate

MFR₂: 190° C., 2.16 kg load

MFR₅: 190° C., 5.0 kg load

MFR₂₁: 190° C., 21.6 kg load

The melt flow rates are measured in accordance with ISO 1133 at 190° C.and under given load and is indicated in units of grams/10 minutes. Themelt flow rate is an indication of the molecular weight of the polymer.The higher the melt flow rate, the lower the molecular weight of thepolymer.

FRR21/5 is a ratio of MFR₂₁/MFR₅

Molecular Weight Averages, Molecular Weight Distribution (Mn, Mw, Mz,MWD, PDI)

Molecular weight averages (Mz, Mw and Mn), Molecular weight distribution(MWD) and its broadness, described by polydispersity index, PDI=Mw/Mn(wherein Mn is the number average molecular weight and Mw is the weightaverage molecular weight) were determined by Gel PermeationChromatography (GPC) according to ISO 16014-1:2003, ISO 16014-2:2003,ISO 16014-4:2003 and ASTM D 6474-12 using the following formulas:

$\begin{matrix}{M_{n} = \frac{\sum\limits_{i = 1}^{N}\; A_{i}}{\sum\limits_{i = 1}^{N}\;( {A_{i}/M_{i}} )}} & (1) \\{M_{w} = \frac{\sum\limits_{i = 1}^{N}\;( {A_{i}{xM}_{i}} )}{\sum\limits_{i = 1}^{N}\; A_{i}}} & (2) \\{M_{z} = \frac{\sum\limits_{i = 1}^{N}\;( {A_{i}{xM}_{i}^{2}} )}{\sum\limits_{i = 1}^{N}\;( {A_{i}/M_{i}} )}} & (3)\end{matrix}$

For a constant elution volume interval ΔV_(i), where A_(i), and M_(i)are the chromatographic peak slice area and polyolefin molecular weight(MW), respectively associated with the elution volume, V_(i), where N isequal to the number of data points obtained from the chromatogrambetween the integration limits.

A high temperature GPC instrument, equipped with either infrared (IR)detector (IR4 or IR5 from PolymerChar (Valencia, Spain) or differentialrefractometer (RI) from Agilent Technologies, equipped with 3×Agilent-PLgel Olexis and 1× Agilent-PLgel Olexis Guard columns was used.As the solvent and mobile phase 1,2,4-trichlorobenzene (TCB) stabilizedwith 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) was used. Thechromatographic system was operated at 160° C. and at a constant flowrate of 1 mL/min. 200 μL of sample solution was injected per analysis.Data collection was performed using either Agilent Cirrus softwareversion 3.3 or PolymerChar GPC-IR control software.

The column set was calibrated using universal calibration (according toISO 16014-2:2003) with 19 narrow MWD polystyrene (PS) standards in therange of 0.5 kg/mol to 11 500 kg/mol. The PS standards were dissolved atroom temperature over several hours. The conversion of the polystyrenepeak molecular weight to polyolefin molecular weights is accomplished byusing the Mark Houwink equation and the following Mark Houwinkconstants:

K_(PS)=19×10⁻³ mL/g, η_(PS)=0.655

K_(PE)=39×10⁻³ mL/g, η_(PE)=0.725

K_(PP)=19×10⁻³ mL/g, η_(PP)=0.725

A third order polynomial fit was used to fit the calibration data.

All samples were prepared in the concentration range of 0.5-1 mg/ml anddissolved at 160° C. for 3 hours for PE under continuous gentle shaking.

Density

Density is measured according to ISO1183-1987

Comonomer Content from PE (NMR)

Quantification of Microstructure by NMR Spectroscopy

Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used toquantify the comonomer content of the polymers.

Quantitative ¹³C{¹H} NMR spectra recorded in the molten-state using aBruker Advance III 500 NMR spectrometer operating at 500.13 and 125.76MHz for ¹H and ¹³C respectively. All spectra were recorded using a ¹³Coptimised 7 mm magic-angle spinning (MAS) probehead at 150° C. usingnitrogen gas for all pneumatics. Approximately 200 mg of material waspacked into a 7 mm outer diameter zirconia MAS rotor and spun at 4 kHz.This setup was chosen primarily for the high sensitivity needed forrapid identification and accurate quantification.{klimke06, parkinson07,castignolles09} Standard single-pulse excitation was employed utilisingthe transient NOE at short recycle delays of 3 s {pollard04, klimke06}and the RS-HEPT decoupling scheme{fillip05,griffin07}.

A total of 1024 (1 k) transients were acquired per spectrum. This setupwas chosen for high sensitivity towards low comonomer contents. When thedetermined comonomer content was observed to be below 0.2 mol % underthese conditions sensitivity was increased by acquiring a total of 16384(16 k) transients per spectrum. This setup was chosen for very highsensitivity towards very low comonomer contents.

Quantitative ¹³C{¹H} NMR spectra were processed, integrated andquantitative properties determined using custom spectral analysisautomation programs. All chemical shifts are internally referenced tothe bulk methylene signal (6+) at 30.00 ppm {randall89}.

Characteristic signals corresponding to the incorporation of 1-hexenewere observed (randall89) and all contents calculated with respect toall other monomers present in the polymer.

Characteristic signals resulting from isolated 1-hexene incorporationi.e. EEHEE comonomer sequences, were observed. Isolated 1-hexeneincorporation was quantified using the integral of the signal at 38.29ppm assigned to the *B4 sites, accounting for the number of reportingsites per comonomer:H=I _(*B4)

With no other signals indicative of other comonomer sequences, i.e.consecutive comonomer incorporation, observed the total 1-hexenecomonomer content was calculated based solely on the amount of isolated1-hexene sequences:H _(total) =H

Characteristic signals resulting from saturated end-groups wereobserved. The content of such saturated end-groups was quantified usingthe average of the integral of the signals at 22.84 and 32.23 ppmassigned to the 2s and 2s sites respectively:S=(½)*(I _(2S) +I _(3S))

The relative content of ethylene was quantified using the integral ofthe bulk methylene (δ+) signals at 30.00 ppm:E=(½)*I _(δ+)

The total ethylene comonomer content was calculated based the bulkmethylene signals and accounting for ethylene units present in otherobserved comonomer sequences or end-groups:E _(total) =E+(5/2)*H+(3/2)*S

The total mole fraction of 1-hexene in the polymer was then calculatedas:fH=(H _(total)/(E _(total) +H _(total))

The total comonomer incorporation of 1-hexene in weight percent wascalculated from the mole fraction in the standard manner:H[wt %]=100*(fH*84.16)/((fH*84.16)+((1−fH)*28.05))

-   klimke06-   Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H. W.,    Wilhelm, M., Macromol. Chem. Phys. 2006; 207:382.-   parkinson07-   Parkinson, M., Klimke, K., Spiess, H. W., Wilhelm, M., Macromol.    Chem. Phys. 2007; 208:2128.-   pollard04-   Pollard, M., Klimke, K., Graf, R., Spiess, H. W., Wilhelm, M.,    Sperber, O., Piel, C., Kaminsky, W., Macromolecules 2004; 37:813.-   filip05-   Filip, X., Tripon, C., Filip, C., J. Mag. Resn. 2005, 176, 239-   griffin07-   Griffin, J. M., Tripon, C., Samoson, A., Filip, C., and Brown, S.    P., Mag. Res. in Chem. 2007 45, S1, S198-   castignolles09-   Castignolles, P., Graf, R., Parkinson, M., Wilhelm, M., Gaborieau,    M., Polymer 50 (2009) 2373-   randall89-   J. Randall, Macromol. Sci., Rev. Macromol. Chem. Phys. 1989, C29,    201.

EXAMPLES Reference Example 1

Preparation of Solid Catalyst Component 1

Mg Compound Preparation:

Toluene (87 kg) was added into the 100 liter reactor. Then Bomag A,provided by Chemtura, (45.5 kg, 20 wt % butyloctyl magnesium in heptane)was also added to the reactor. Then 2-ethyl-1-hexanol (161 kg, 99.8 wt%) was introduced into the reactor at a flow rate of 24-40 kg/h. Themolar ratio between BOMAG-A and 2-ethyl-1-hexanol was 1:1.83.

Preparation of Solid Catalyst Component:

Silica (330 kg of calcined silica, Sylopol® 2100) and pentane (0.12kg/kg carrier) were charged into a catalyst preparation reactor. ThenEADC (ethylaluminium dichloride, 2.66 mol/kg silica) was added into thereactor at a temperature below 40° C. during two hours and mixing wascontinued for one hour. The temperature during mixing was 40-50° C. Thenthe magnesium compound prepared as described above was added (2.56 molMg/kg silica) at 50° C. during two hours and mixing was continued at40-50° C. for one hour. Then 0.84 kg pentane/kg silica was added intothe reactor and the slurry was stirred for 4 hours at the temperature of40-50° C. Finally, TiCl₄ (1.47 mol/kg silica) was added during at least1 hour, but less than 1.5 hours at 55° C. to the reactor. The slurry wasstirred at 50-60° C. for five hours. The catalyst was then dried bypurging with nitrogen.

Molar composition of the recovered catalyst is: Al/Mg/Ti=1.5/1.4/0.8

Reference Example 2

Preparation of Solid Catalyst Component 2

Raw Materials

The standard 10 and 25 wt % TEA (triethyl aluminium) solutions inheptane were prepared by dilution of 100% TEA-S from Chemtura.

MgCl₂*3EtOH carriers were received from GRACE

2,2-Di(2-tetrahydrofuryl)propane (DTHFP) was supplied by TCI EUROPE N.V.as a mixture (1:1) of diastereomers (D,L-(rac)-DTHFP and meso-DTHFP.

TiCl₄ was supplied by Aldrich (Metallic impurities<1000 ppm, Metalsanalysis>99.9%).

A. Pre-Treated Support Material Preparation:

In an inert atmosphere glove box (<1 ppm O₂, H₂O): A dry 100 mL, 4-neckround-bottom flask equipped with two rubber septa, a thermometer, andmechanical stirrer was charged with 0.38 g of DTHFP (DTHFP/Mg=0.1mol/mol) dissolved in 30 mL of heptane and 5 g (20 mmol of Mg) ofgranular 21 μm (d50) MgCl₂*3EtOH carrier. The flask was removed from theglove-box, a nitrogen inlet and outlet were fixed. The flask was placedin a cooling bath and stirred for approximately 10 min at 250 rpm. Aprecooled 25 wt % solution of triethylaluminum (30.4 g, 67 mmol of Al;Al/EtOH=1.0 mol/mol) in heptane was added dropwise during 1 h time,keeping the temperature below 0° C. The obtained suspension was heatedto 80° C. in 20 min and kept at this temperature for 30 min at 250 rpm.The suspension was settled for 5 min at 80° C., and the liquid wasremoved via cannula. The obtained pre-treated support material waswashed twice with 50 mL of toluene at room temperature (addition oftoluene, stirring at 250 rpm for 15-120 min, settling for 5 min, removalof liquid by cannula).

B. Catalyst Preparation:

At room temperature, 50 mL of toluene was added to the pre-treatedsupport material. The mixture was stirred for approximately 30 min at250 rpm. Neat TiCl₄ (3.8 g, 20 mmol; Ti/Mg=1.0 mol/mol) was addeddropwise, and the temperature was maintained between 25-35° C. Theobtained suspension was heated to 90° C. in 20 min and kept at thistemperature for 60 min at 250 rpm. The suspension was settled for 5 minat 90° C., and the liquid was removed via cannula. The obtained catalystwas washed twice with 50 mL of toluene at 90° C., and once with 50 mL ofpentane at room temperature (addition of preheated toluene or pentane,stirring at 250 rpm for 15 min, settling for 5 min, removal of liquidvia cannula). The catalyst was dried with nitrogen flow at 50° C. for1.5 h. The yield was 3.4 g (94% based on Mg).

Inventive Examples 1-5 (IE1-IE5) and Comparative Examples 1, 2 and 3(CE1, CE2 and CE3)

Ethylene hexene copolymer was produced in a continuous multistageprocess comprising two slurry-loop reactors of size 150 and 350 litersand one gas phase reactor. In addition a prepolymerisation step was usedin examples IE4-IE5 and CE1 and CE2. Temperature in theprepolymerisation step was 70° C., 950° C. in the loop reactors, and 85°C. in the gas phase reactor. In examples IE1, IE2 and IE3 and CE3 theprepolymerisation step was not used. Propane was used as the reactionmedium in the loop reactors. The same catalyst component of referenceexample 1 and triethylaluminium (TEA) as cocatalyst were used at anAl/Ti molar ratio of 2. The sum of all cocatalyst feeds to the loopreactor includes the optional prepolymerisation step, 1^(st) loopreactor, and 2^(nd) loop reactor. In the inventive examplesdimethyldimethoxy silane (DMDS) was used as the external additive(external donor) supplied by TCI EUROPE N.V., used as received.

In comparative examples no external donor was used.

The polymerisation conditions and results of IE1-IE5 and CE1-CE3 aredisclosed in Table 1 and properties of the corresponding final polymersare disclosed in Table 2.

TABLE 1 Polymerisation conditions and final polymer properties EXAMPLEIE1 IE2 IE3 IE4 IE5 CE1 CE2 CE3 Catalyst feed 5.2 5.0 5.1 6.0 9.0 10.49.0 9.6 (g/h) PREPOL REACTOR C₂ feed (kg/h) — — — 2.0 2.0 2 2 — H₂ feed(g/h) — — — 4.8 4.9 4.9 4.8 — 1^(st) LOOP REACTOR (A1) Press. (MPa) 6.06.0 6.0 6.0 6.0 5.6 5.6 6.0 H₂/C₂ ratio 496 488 503 288 485 421 269 338(mol/kmol) split % 32 30 28 29 24 23 24 21 MFR₂ (g/10 min) 504 513 516105 402 224 85 295 2^(nd) LOOP REACTOR (A2) Press. (MPa) 5.4 5.4 5.4 5.55.5 5.2 5.2 5.4 H₂/C₂ ratio 437 441 440 326 486 534 411 369 (mol/kmol)split % 34 31 29 30 25 23 25 32 MFR₂ (g/10 min) 371 383 384 109 464 488170 295 Al/Ti (mol/mol) 15 16 16 14 15 6 7 10 GAS PHASE REACTOR (B)Temp. (° C.) 85 85 85 85 85 85 85 85 Press. (MPa) 2.0 2.0 2.0 2.0 2.02.0 2.0 2.0 DMDS feed (g/h) 0.90 0.90 0.90 0.55 0.63 — — — C₂ kPapartial 406 558 496 626 530 124 128 164 Si/Ti ratio 1.9 2.0 2.0 1.0 0.8— — — (mol/mol) H₂/C₂ 1.8 2.7 1.4 5.2 14.5 3.2 4.3 3.1 ratio (mol/kmol)C₆/C₂ ratio 99 41 48 49 20 54 53 33 (mol/kmol) split % 34 39 43 41 48 5149 46 Cat. prod. kg 15.9 17.1 17.4 13.2 9.6 7.9 8.5 9.1 PE/g cat

TABLE 2 Properties of the final polymer EXAMPLE IE1 IE2 IE3 IE4 IE5 CE1CE2 CE3 Density (kg/m3) 953.2 955.3 954.0 953.9 954.3 947.0 949.7 954.9MFR₅ (g/10 min) 0.23 0.07 0.09 0.16 0.20 0.27 0.36 0.16 MFR₂₁ (g/10 min)14.70 5.81 6.50 9.30 8.65 9.35 11.70 6.15 FFR_(21/5) 64 83 72 58 43 3532 38 C₆ content by ¹³C nm nm 0.9 0.6 0.7 1.8 1.5 0.4 NMR (wt %) Mn nmnm 9430 11100 9335 9630 11250 9975 Mw nm nm 406500 318500 248500 239000248000 257500 Mz nm nm 2500000 1910000 1335000 1435000 1565000 1515000PDI (Mw/Mn) nm nm 43 29 27 25 22 26 nm = not measured

Inventive Example 6 (IE6) and Comparative Example 4 (CE4)

Ethylene hexene copolymer was produced in a continuous multistageprocess comprising two slurry-loop reactors of size 150 and 350 litersand one gas phase reactor. In addition a prepolymerisation step was usedin examples IE6 and CE4. Temperature in the prepolymerisation step was70° C., 950° C. in the loop reactors, and 85° C. in the gas phasereactor. Propane was used as the reaction medium in the loop reactors.The same catalyst component prepared according to the reference example2 and triethylaluminium (TEA) as cocatalyst were used at an Al/Ti molarratio of 2. The sum of all cocatalyst feeds to the loop reactor includesthe optional prepolymerisation step, 1^(st) loop reactor, and 2^(nd)loop reactor. In the inventive example IE6 dimethyldimethoxy silane(DMDS) was as the external additive (external donor), supplied by TCIEUROPE N.V., used as received.

In comparative example CE4 no external donor was used.

The polymerisation conditions, results and final polymer properties ofIE6 and CE4 are disclosed in Table 3

TABLE 3 EXAMPLE IE6 CE4 Catalyst feed (g/h) 10.9 14.9 PREPOL REACTOR 7070 C₂ feed (kg/h) 2.0 2.0 H₂ feed (g/h) 2 5 1^(st) LOOP REACTOR (A1)Press. (MPa) 5.6 5.6 H₂/C₂ ratio (mol/kmol) 601.4 629.7 split % 21 20MFR₂ (g/10 min) 112 274 2^(nd) LOOP REACTOR (A2) Press. (MPa) 5.1 5.1H₂/C₂ ratio (mol/kmol) 427 650 split % 38 37 MFR₂ (g/10 min) 188 388Al/Ti (mol/mol) 15 15 GAS PHASE REACTOR (B) Temp. (° C.) 85 85 Press.(MPa) 2.0 2.0 DMDS feed (g/h) 3.64 — C₂ kPa partial 2.6 0.54 Si/Ti ratio(mol/mol) 2.7 — H₂/C₂ ratio(mol/kmol) 2.2 11.7 C₆/C₂ ratio (mol/kmol)7.0 18.9 split % 40 41 Cat. prod. kg PE/g cat 7.1 5.7 FINAL POLYMERDensity (kg/m3) 954.8 954.3 MFR₅ (g/10 min) 0.06 0.24 MFR₂₁ (g/10 min)4.72 14.90 FFR_(21/5) 79 62 C₆ content by ¹³C NMR (wt %) 0.25 0.6 Mn8595 6970 Mw 496500 279500 Mz 2410000 1640000 PDI (Mw/Mn) 58 40

As can be seen from the examples, ethylene copolymers produced with theprocess and with the catalysts of the invention containing definedalkoxysilane as external additive have lower MFR₅ combined with highFFR_(21/5) values and thus clearly higher molecular weight of thefraction of the gas phase reactor (GPR) than ethylene copolymersproduced with a catalyst without any external donor. Definedalkoxysilanes as external additives reduce hydrogen response of thecatalysts (IE5 vs. CE1-3) and also increase ethylene partial pressure inGPR (IE1-3 vs. CE1-3 and IE6 vs. CE4), thus allowing production ofethylene copolymer of higher molecular weight in GPR, than with acatalyst without any external donor (FIGS. 1 and 2). Further, the higherFFR_(21/5) ratios of the inventive examples indicate that the desiredbroader molecular weight distribution is achieved. At the same time theproductivity of the inventive catalysts comprising the defined externaladditives is higher (FIG. 3).

The invention claimed is:
 1. A multistage process comprising at leastone slurry phase polymerization stage and at least one gas phasepolymerization stage for producing ethylene copolymers comprising thesteps of (a1) introducing ethylene, optionally hydrogen and optionallyalpha-olefin comonomer having from 4 to 10 carbon atoms into an optionalpolymerization stage A1 in the presence of a solid catalystZiegler-Natta component, a cocatalyst and optionally an externaladditive, (b1) maintaining said polymerization stage A1 in suchconditions as to produce an ethylene homo- or copolymer product P-A1(a2-i) feeding ethylene, the polymerization product P-A1, optionallyalpha-olefin comonomer having from 4 to 10 carbon atoms and optionallyan external additive to a polymerization stage A2, or (a2-ii) feedingethylene, a solid catalyst Ziegler-Natta component, a cocatalyst,optionally alpha-olefin comonomer having from 4 to 10 carbon atoms andoptionally an external additive to a polymerization stage A2 (b2)maintaining said polymerization stage A2 in such conditions as toproduce a lower molecular weight (co)polymer P-A2 or a (co)polymermixture P-M1 comprising the optional ethylene (co)polymer P-A1 and thelower molecular weight ethylene (co)polymer P-A2, (c) feeding thepolymerization product P-A2 or the (co)polymer mixture P-M1, additionalethylene and an alpha-olefin comonomer having from 4 to 10 carbon atoms,an external additive, which can be the same or different as the optionalexternal additive in step (a1) or (a2), optionally hydrogen andoptionally additional cocatalyst to the polymerization stage B (d)maintaining said polymerization stage B in such conditions as to producea higher molecular weight polymerization product P-B, (e) recovering thepolymerization product P-B from the polymerization stage B, wherein theexternal additive has formula (I)R¹ _(n)Si(OR²)_(4-n),  (I) where n is an integer from 0 to 3, each R¹are equal or different and are selected from H, halogen, alkyl groups of1 to 6 carbon atoms optionally substituted with one or more halogenatoms, alkenyl groups of 2 to 6 carbon atoms optionally substituted withone or more halogen atoms, and aryl groups of 6 to 12 carbon atomsoptionally substituted with one or more halogen atoms, or the R¹ groupscan form with the Si atom they are linked to a ring of 3 to 8 ringatoms, provided that all R¹ are not hydrogen, each R² are equal ordifferent and are selected from alkyl groups of 1 to 6 carbon atomsoptionally substituted with one or more halogen atoms, alkenyl groups of2 to 6 carbon atoms optionally substituted with one or more halogenatoms, and aryl groups of 6 to 12 carbon atoms optionally substitutedwith one or more halogen atoms, or the OR² groups can form with the Siatom they are linked to a ring of 3 to 8 ring atoms, halogen is Br, Clor F, and wherein the polymerization stage B is a gas phasepolymerization stage.
 2. The multistage process according to claim 1,wherein the polymerization product P-B is an ethylene copolymer having amelt flow rate ratio FRR_(21/5) of at least 40 and/or a polydispersityindex PDI of at least
 27. 3. The multistage process according to claim1, wherein in the formula (I) n is an integer from 1 to 3, all R¹ groupsare the same or different and are hydrogen or alkyl groups of 1 to 6carbon atoms or aryl groups of 6 to 12 carbon atoms, provided that allR¹ are not hydrogen, and all R² groups are the same and are alkyl groupsof 1 to 3 carbon atoms.
 4. The multistage process according to claim 1,wherein each R¹ is independently hydrogen, methyl or ethyl, providedthat at least one R¹ is methyl or ethyl, R² is methyl or ethyl, and n is1 or
 2. 5. The multistage process according to claim 1, comprisingpolymerization stages A1, A2 and B, and wherein each polymerizationstage A1 and A2 is carried out in a slurry reactor, and stage B iscarried out in one gas phase reactor.
 6. The multistage processaccording to claim 1, wherein the external additive is fed to thepolymerization process in an amount corresponding to a Si/Ti mol/molratio of from 0.2 to 5.0.
 7. The multistage process according to claim1, wherein the external additive is fed only to the polymerization stageB.
 8. The multistage process according to claim 1, wherein the comonomeris selected from 1-butene, 1-hexene, and mixtures thereof.
 9. Themultistage process according to claim 1, wherein the solid catalystZiegler-Natta component comprises a compound of a Group 4 to 6 metal,optionally an aluminum compound, optionally an internal organic compoundand a magnesium compound supported on an inorganic oxide support. 10.The multistage process according to claim 1, wherein the solid catalystZiegler-Natta component comprises a compound of a Group 4 to 6 metal,optionally an aluminum compound and optionally an internal organiccompound supported on a magnesium dichloride support.
 11. The multistageprocess according to claim 10, wherein the solid catalyst Ziegler-Nattacomponent comprises a solid MgCl₂ supported component which is preparedby a method comprising the steps of: a) providing solid carrierparticles of MgCl₂*mROH adduct; b) pre-treating the solid carrierparticles of step a) with a compound of a Group 13 metal; c) treatingthe pre-treated solid carrier particles of step b) with a transitionmetal compound of Group 4 to 6; and d) recovering the solid catalystcomponent; wherein the solid carrier particles are contacted with aninternal organic compound of formula (II) or isomers or mixturestherefrom before treating the solid carrier particles in step c)

and wherein in the formula (II) or isomers or mixtures therefrom R₁ toR₅ are the same or different and can be hydrogen, a linear or branchedC₁ to C₈-alkyl group, or a C₃-C₈-alkylene group, or two or more of R₁ toR₅ can form a ring, the two oxygen-containing rings are individuallysaturated or partially unsaturated or unsaturated, and R in the adductMgCl₂*mROH is a linear or branched alkyl group with 1 to 12 C atoms, andm is 0 to
 6. 12. The multistage process according to claim 1, whereinthe external additive is selected from the group consisting ofdimethoxydimethylsilane, trimethoxymethylsilane, diethoxydimethylsilane,dimethoxydiethylsilane, dimethoxydi-n-propylsilane,dimethoxy(methyl)silane, vinylmethyldimethoxysilane,chloromethyl(methyl)dimethoxysilane, dimethoxymethylphenylsilane,3-chloropropyldimethoxymethylsilane,trimethoxy(3,3,3-trifluoropropyl)silane, and3-chloropropyltrimethoxysilane.
 13. The multistage process according toclaim 1, wherein the cocatalyst is an organometal compound of a Group 13metal.
 14. The multistage process according to claim 1, wherein thepolymerization product P-B is an ethylene copolymer having a melt flowrate ratio FRR_(21/5) of at least 40 and a polydispersity index PDI ofat least
 27. 15. The multistage process according to claim 1, whereinthe external additive is fed to the polymerization process in an amountcorresponding to a Si/Ti mol/mol ratio of from 0.5 to 2.5.
 16. Themultistage process according to claim 1, wherein polymerization steps A1and A2 are carried out in a loop reactor.
 17. The multistage processaccording to claim 1, wherein the solid catalyst Ziegler-Natta componentcomprises a titanium compound.